Posted by **Karla** on Monday, August 29, 2011 at 5:37am.

Hey :)

I was wondering if someone could tell me if this answer and working is correct or not...

Cos theta =-0.25

Cos inverse -0.25 = -14

therefore theta equals:

180--14=194or/and

180+-14=166

Any help would be much appreciated :)

Thanks!

- Maths -
**Jai**, Monday, August 29, 2011 at 6:10am
cos (theta) = -0.25

theta = cos^-1 (-0.25)

there are two answers:

theta = 104.48 degrees = 3.65 radians

and,

theta = -104.48 degrees = -3.65 radians

hope this helps~ :)

- Maths -
**Reiny**, Monday, August 29, 2011 at 8:30am
Usually the required angles are in the domain of

0 ≤ Ø ≤ 360°

Since the given cosØ is negative, by the CAST rule,

Ø must be in either quadrant II or III

from cosØ = +.25 we get the reference angle of 75.52°

so Ø = 180°-75.52° or 104.48° (Jai's answer)

or Ø = 180°+75.52° or 255.52°

Since the period of cosØ is 360°, you can get other answers by adding or subtracting multiples of 360°

(For Jai's second answer, 255.52 - 360° = 104.48°)

I have no idea how you came up with

inverse cos (-.25) = -14

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